1. Field of the Invention
The present invention relates generally to optical switches, and particularly to wavelength selective switches using a polarization rotating device.
2. Technical Background
In the past two-decades fiber optics have transformed the telecommunications market place. Initially, network designs included relatively low-speed transceiver electronics at each end of the communications link. Light signals were switched by being converted into electrical signals, switched electronically, and reconverted into light signals. The bandwidth of electronic switching equipment is limited to about 10 GHz. On the other hand, the bandwidth of single mode optical fiber in the 1550 nm region of the electromagnetic spectrum is in the Terahertz range. As the demand for bandwidth increases exponentially, network designers have sought ways to exploit the available bandwidth in the 1550 nm region. Thus, a need exists for optically transparent cross-connects and switches.
One approach that has been considered involves a frequency-selective optical switch employing a polarization beam splitter, Wollaston prism and a liquid crystal switch element. However, this design has a major drawback. The polarizing beam splitter, which is used to recombine the beams, is always located between the focusing lens and the spatial light modulator. One effect of this is that the polarizing beamsplitter must be able to accept a large acceptance angle, which leads to poorly superimposed beams if birefringent crystals are used. If beamsplitting cubes are used contrast ratio is reduced and crosstalk is increased. This was addressed by using a Wollaston Prism. Wollaston Prisms are designed to convert a collimated beam of mixed polarization into two deflected collimated beams, which are separated by an angle that is roughly bisected by the optical axis of the original mixed polarization beam. This solves many of the problems associated with placing the polarizing beam separator between the focusing lens and the LC switch element, but there are substantial problems associated with using Wollaston Prisms. The most significant of these lies is the fact Wollaston Prisms cannot produce beams that are exactly symmetrically deflected. Because the effect of the Wollaston Prism is not symmetrical, the beams cannot be superimposed at the LC switch element. Thus, the positions of the beams at the LC switch element must be balanced with the differing angles of incidence at the LC switch element to minimize crosstalk and insertion loss variation for the different switched states. Due to this asymmetry, the optical system must grow to unattractively long lengths in order to achieve acceptable crosstalk with an acceptable channel bandwidth.
Thus, what is needed is a wavelength selective switch having an optical system that is symmetric about a polarization modulator and capable of delivering superimposed beams at the polarization modulator in order to reduce crosstalk, reduce insertion loss, and improve spectral resolution.